With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there is an increasing demand for secondary batteries having a small size, a light weight and a high energy density as a power source for driving these electronic devices. Also, in consideration of global environments, electric vehicles and hybrid vehicles have been recently developed and put into practice, so that there is an increasing demand for lithium ion secondary batteries for large size applications having excellent storage characteristics. Under these circumstances, the lithium ion secondary batteries having advantages such as a large charge/discharge capacity and good storage characteristics have been noticed.
Hitherto, as positive electrode active substances useful for high energy-type lithium ion secondary batteries having a 4 V-grade voltage, there are generally known LiMn2O4 having a spinel structure, LiMnO2 having a zigzag layer structure, LiCoO2 and LiNiO2 having a layer rock-salt structure, or the like. Among the secondary batteries using these active substances, lithium ion secondary batteries using LiNiO2 have been noticed because of a large charge/discharge capacity thereof. However, the materials tend to be deteriorated in thermal stability upon charging and charge/discharge cycle durability, and, therefore, it has been required to further improve properties thereof.
Specifically, when lithium is extracted from LiNiO2, the crystal structure of LiNiO2 suffers from Jahn-Teller distortion since Ni3+ is converted into Ni4+. When the amount of Li extraction reaches 0.45, the crystal structure of such a lithium extraction region of LiNiO2 is transformed from hexagonal system into monoclinic system, and a further extraction of lithium therefrom causes transformation of the crystal structure from monoclinic system into hexagonal system. Therefore, when the charge/discharge reaction is repeated, the crystal structure of LiNiO2 tends to become unstable, so that the resulting secondary batteries tend to be deteriorated in cycle characteristics or suffer from occurrence of undesired reaction between LiNiO2 and an electrolyte solution owing to release of oxygen therefrom, resulting in deterioration in thermal stability and storage characteristics of the battery. To solve these problems, studies have been made on those materials produced by adding Co, Al, Mn and the like to LiNiO2 to substitute a part of Ni in the LiNiO2 therewith. However, the materials have still failed to solve the aforementioned problems. Therefore, it has still been required to provide an Li-Ni composite oxide having a more stabilized crystal structure.
In addition, the Li-Ni composite oxide particles are constituted of primary particles having a small particle diameter. Therefore, in order to obtain an Li-Ni composite oxide having a high packing density, it is necessary to suitably control properties of the Li-Ni composite oxide such that the primary particles are densely aggregated together to form secondary particles thereof. However, the thus formed secondary particles of the Li-Ni composite oxide tend to be broken by compression upon production of an electrode therefrom, so that the Li-Ni composite oxide tends to suffer from increase in surface area and accelerated reaction with an electrolyte solution upon storage in high-temperature charged conditions to form an insulator film along a boundary surface of the electrode and thereby raise a resistance of the resulting secondary battery. Also, the Li-Ni composite oxide tends to undergo initiation of decomposition reaction that is accompanied with release of oxygen even at a low temperature as compared to Li-Co composite oxide, so that there is such a fear that the released oxygen causes combustion of the electrolyte solution, and the resulting battery suffers from rapid increase in temperature or explosion. Under these circumstances, in order to improve a thermal stability of the Li-Ni composite oxide upon storage under high-temperature conditions, it is necessary to efficiently increase a crystallite size (primary particle diameter) thereof to such an extent that the Li-Ni composite oxide is free of deterioration in a discharge capacity, and suppress the reaction of the Li-Ni composite oxide with the electrolyte solution or stabilize a crystal structure thereof.
That is, there is an increasing demand for Li-Nicomposite oxide that can exhibit a high discharge capacity as a positive electrode active substance for non-aqueous electrolyte secondary batteries and is excellent in thermal stability.
Hitherto, in order to increase a capacity, control a crystallite size, stabilize a crystal structure and improve various properties such as a thermal stability, various improvements in LiNiO2 particles have been attempted. For example, there are known the technology in which a composition of an Li-Ni composite oxide from which Li is withdrawn by charging is controlled such that the content of tetravalent Ni therein is not more than 60% to improve a thermal stability thereof (Patent Literature 1); the technology in which a part of Ni in an Li-Ni composite oxide is substituted with at least one element selected from the group consisting of metal species including Co, Al and Mn, and after calcining the Li-Ni composite oxide, an excessive amount of Li is removed therefrom to improve cycle characteristics, a thermal stability and storage characteristics thereof (Patent Literature 2); the technology in which an oxide of at least one element selected from the group consisting of B and P is incorporated into an Li-Ni composite oxide to control a crystalline size of the composite oxide and improve a thermal stability thereof (Patent Literature 3); the technology in which a part of Ni in an Li-Ni composite oxide is substituted with Co and Al to stabilize a crystal structure thereof (Patent Literature 4); and the like.